Liquid ring vacuum pumps are suitable for moving fluids. Such fluids will be referred to as “working fluids” throughout the description. The fluid may be a liquid fluid and/or a gaseous fluid. Liquid ring vacuum pumps are commonly used to transfer working fluids in drug, food and plastic extrusion plants. For example, a vacuum pump may be used to evacuate working fluid that has been used to cool hot extruded plastic. In this particular example, the extracted fluid is usually a mixture of air and water. A liquid ring vacuum pump is depicted in FIGS. 1 and 2. FIG. 1 depicts a stationary pump and FIG. 2 depicts a pump in operation. The pump (1) includes a housing (2), which is partially filled with a barrier fluid (3). A multi-bladed impeller (4) is mounted on a shaft (5). The shaft is arranged off-centre from the central axis of the housing (2) such that the impeller is positioned eccentrically. Port plates (not shown) are arranged on either side of the impeller (not shown) forming an inlet and outlet into the housing. Thus, one side of the pump is known as the inlet port side and the other side of the pump is known as the outlet port side. As the impeller rotates a centrifugal force pushes the barrier fluid towards the periphery of the housing. A ring (6) of barrier fluid is formed around the inner wall of the pumping chamber with a constant width and depth. Since the impeller (4) is mounted eccentrically, the depths to which the blades penetrate the liquid ring (6) vary as the impeller (4) rotates. It can be seen in FIG. 2 that the depth of penetration of any given blade of the impeller (4) decreases as the blade rotates from an upper position indicated at (A) to lower position indicated at (B). This leads to an increase in the impeller cell volume (the space formed between the liquid ring and root of the blade) that in turn leads to the creation of a vacuum. Meanwhile, the depth of penetration increases as the blades rotate from the lower position to the upper position. Thus, the impeller cell volume decreases and so pressure increases. In operation, the vacuum draws working fluid in through the inlet and into the cavity formed between the root of the impeller blade and inside diameter of the liquid ring and the high pressure region subsequently discharges the working fluid through the outlet.
The barrier fluid is an essential component of a liquid ring vacuum pump. The barrier fluid is required to lubricate the pump, help create a vacuum so that working fluids are drawn into the pump, help create a high pressure region in order to discharge working fluids from the pump and also cool the pump.
The barrier fluid of a liquid ring vacuum pump includes at least one liquid fluid and optionally at least one gaseous fluid. Water is typically used as a barrier fluid. However, other suitable liquids may be used as an alternative.
During operation, barrier fluid is also drawn into the vacuum pump through the inlet and discharged through the outlet. The volume of barrier liquid drawn into the pump is generally equal to the volume of barrier fluid discharged from the pump. The discharged evacuated fluid may be separated from the discharged barrier fluid. It is imperative that the liquid ring vacuum pump operates using the correct amount of barrier fluid. The correct quantity of barrier fluid may vary between different designs of pumps but, for many common pump designs, when the pump is stationary, the volume of barrier fluid should be such that the barrier fluid level reaches the centre line of the pump. Again, the exact level may vary between different pump designs: in some pumps the barrier fluid level is just above the centre line of the pump and in other pump designed it may be just below. Thus for any given pump design there is a predetermined optimum fill level for the barrier fluid which is required for efficient operation of the pump. If the pump has too much barrier fluid then it will seek to compress the incompressible fluids. This may cause the blades of the impeller to bend and possibly break. Pump failure may also occur if the pump has too little fluid. The pump will be unable to create a vacuum, the pump may run dry or cavitation may occur which may subsequently cause damage to the internal structure of the pump.
Unfortunately, the volume of barrier fluid within the pump may vary. The volume of barrier fluid may vary gradually over time or change suddenly due a process upset. For example, barrier fluid may leak from the pump. Barrier fluid may evaporate if it becomes too hot. Also, sudden changes in temperature and/or pressure may result in either a loss or increase in barrier fluid. Thus, a system for controlling the volume of barrier fluid in a liquid ring vacuum pump is required.
A known system for regulating the volume of barrier fluid in a liquid ring vacuum pump is depicted in FIG. 3. The system includes a chamber (7) containing a quantity 7′ of barrier fluid. The chamber (7) is connected to the inlet (1A) and outlet (1B) of the pump (1). Thus, barrier fluid is discharged from the pump into the chamber and drawn from the chamber into the pump. A return valve (8) ensures that barrier fluid cannot leave the pump (1) from the wrong exit (9). The chamber (7) is also connected to a barrier fluid source (11) in order to replenish any barrier fluid that is lost due to leaks, evaporation and/or process upsets. The replenishing source of barrier fluid is controlled by a regulating valve (11A). The chamber (7) is connected to an outlet (12) that enables any excess barrier fluid to drain away. Any gas discharged into the chamber (7) is released through a vent (10). A shell and tube heat exchanger (13) is arranged between the chamber (7) and pump (1) in order to cool the barrier fluid prior to re-entry into the pump (1). The heat exchanger (13) uses cooling liquid (14) to help cool the barrier fluid. A return valve (15) is arranged to control the re-entry of barrier fluid into the vacuum pump (1).
The regulating valve (11A) for the barrier fluid source is manually controlled. Thus, the barrier fluid is replenished using trial and error and this often leads to an over-supply or under-supply of barrier fluid in the vacuum pump (1). A manually controlled valve is also prone to human error. Furthermore, the regulating valve (11A) requires continual adjustment whilst the pump (1) is in operation. Therefore, the pump (1) has a high risk of failing. Typically, adjustment of the barrier fluid is reliant on the judgement of an experienced operator who can judge from, for example, the sound of the pump whether replenishment of the barrier fluid is necessary. Visual inspection of the level of barrier fluid in the vessel (7) is not carried out for determining whether the quantity of barrier fluid contained in the system is correct.
As explained above, it is essential that the pump (1) has the correct volume of barrier fluid. Therefore, a more accurate means and method of adjusting the volume of barrier fluid is required. In the prior art this has been achieved by adding a flow indicator to the regulating valve (11A) and adding a flow indicator to the outlet (1B) from the pump (1). The flow indicators electronically detect and control the flow of fluid. The flow indicators are centrally controlled such that the flow of barrier fluid from the source (11) is adjusted in accordance with the flow of barrier fluid from the outlet of the pump (1B). However, this solution is costly and difficult to implement, it unduly increases the complexity of the system and produces a system that is both costly and difficult to maintain. Therefore, an accurate, relatively simple and relatively cheap means and method of regulating the volume of barrier fluid in a liquid ring vacuum pump is required.